c - 为什么这个循环每次循环打印相同的值？

Question

这个问题已经有答案了:

srand function is returning same values (1 个回答)

已关闭 7 年前。

我在 main() 中有以下循环

while (count < 5){
            //with 2^15 as the lower limit
                srand(time(NULL));
                ran_num = rand() % (65536 - 32769); //check READ ME Citation A
                ran_num += 32769;
                binary = Integer_to_Binary(ran_num);
                count += Primality_Test(ran_num, binary, sizeof(binary));
            }

printf声明位于 Primality_Test功能...

int Primality_Test(unsigned __int128 ran_num, int binary[], int num_bits){
    int result = Modular_Exponentiation(ran_num, binary, num_bits);
    if (result == 1){
        //the number is prime
        printf("[%d] passes the primality test as prime\n", ran_num); //print binary array too
        return 1;
    }
    else{
        //call Charmichael_Test
        result = Charmichael_Test(ran_num);
        //check for errors
        if (result == 1){
            //number is prime
            printf("[%d] is a charmichael number\n", ran_num);
            return 1;
        }
        else {
            //number is not prime
            //printf("This number is not prime");
            return 0;
        }
    }

}

即使在调试时，ran_num 正在更改，我的输出始终是相同的数字

[57221] is a charmichael number
[57221] is a charmichael number
[57221] is a charmichael number
[57221] is a charmichael number
[57221] is a charmichael number

Answer 1

因为你在循环中调用了srand()。您使用 time(NULL) 给出以秒为单位的时间，因此 5 个循环中每个循环的值可能相同。由于 srand() 设置 rand() 序列的“起始”点，因此您具有相同的值。

将 srand() 提取到循环之外。

请注意，如果循环的值非常大，您将看到每秒打印值的变化。